Electrical and thermal properties of La0.2Sr0.8Cu0.1Fe0.9O3-δ and La0.2Sr0.8Cu0.2Fe0.8O3-δ

La0.2Sr0.8Cu0.1Fe0.9O3-delta and La0.2Sr0.8Cu0.2Fe0.8O3-delta both exhibit cubic perovskite structures with lattice parameters a = 0.38711 and 0.38694 nm, respectively. The initial oxygen non-stoichiometry, delta, of the as-synthesized materials was 0.24 for the sample with 10% Cu and 0.34 for 20% Cu. La0.2Sr0.8Cu0.1Fe0.9O3-delta has a lower p(O2) stability limit at 1 X 10(-4) atm, whereas La0.2Sr0.8Cu0.2Fe0.8O3-delta starts to decompose at 1x10(-3) atm. The conductivity is similar to 10 S cm(-1) and it decreases with decreasing p(O2) from 1 atm to the lower p(O2) stability limit, following a 1/4 power p(O2) dependence for temperatures above 800 K. The activation energy in O-2 is 1-2 kJ mol(-1) and this increases to about 10 kJ mol(-1) for the lower p(O2) limit. The Seebeck coefficient is very small, usually within +/-15 mu V K-1 of zero. Below 650 K, the weight is constant with temperature, whereas above 750 K delta increases with increasing temperature, causing an increase in thermal expansion. The thermal expansion coefficient below 650 K is approximately 15 x 10(-6) K-1, and approximately 30X10(-6) K-1 above 750 K. (C) 2000 Elsevier Science B.V. All rights reserved.